Windshear has always been a serious hazard in the aviation field as is evidenced by disastrous commercial aircraft crashes resulting in a tragic loss of life. The real danger is not when an aircraft is in flight. Rather the hazard is encountered when windshear occurs close to the ground, during landing approaches or on takeoff, that a crash can result. There are both angular and vertical windshears but they are related. Windshear occurs when cold upper air, sometimes with rain, gusts downward because of its heavier density in relation to lighter earth-warmed air at low elevations. Air has been found to gust down vertically at as much as 4000 feet per minute from distances of 2000 feet from the ground. As the gust reaches the ground it has to change direction and in an aircraft landing approach it becomes two horizontal winds rushing out in opposite directions. An aircraft flying through it first encounters a headwind providing good lift. It then in a very short space of time or distance encounters the opposite wind which is moving with the aircraft or a tail wind. If an aircraft is flying flaps down and near landing speed and encounters the tailwind, the pilot experiences a sudden loss of airspeed. If adjustments cannot be made quickly enough or if the plane is so heavily loaded or draggy that it cannot make rapid accommodations, it may crash. An example of this was Delta flight 191 crashing in August of 1985 at the Dallas airport killing 136 people.
Cold wind sometimes descends downwardly at an angle rather than strictly vertically. As the gusting wind reaches the earth it has to turn and its velocity is greatest in a horizontal direction away from the angle of descent. If the angle is steep, for example 30 degrees from the vertical, there will be some resultant back wind but, again, the horizontal wind velocity in the direction of the descent will be greatest. A dramatic instance of this occurred in Denver in 1975. The airplane began its takeoff with a quartering headwind of 10 to 20 knots. It then flew into a tailwind estimated at 60 to 90 knots. Clearly, power could not be applied fast enough and in about five seconds the airspeed decreased from 157 to 116 knots. The crew was unable to keep the craft aloft because of their inability to react quickly enough.
The short distance and brief time period within which windshears may be encountered has led to several different approaches to detect them.
On board instrumentation for detecting windshear on large commercial aircraft has been developed, but again warning of windshear conditions is given only a few seconds in advance of encountering the condition. Known instruments are not designed for small craft and airports do not provide windshear information. In short, no adequate system for detecting and providing windshear information to aircraft has been designed of which applicant is aware.
As a practical matter systems for real time detection systems currently in use at airports report only variations in wind velocity at the tower and not in approach areas Hence, the vagueness of tower warnings of possible windshear conditions.
The only art of which applicant is aware includes U.S. Pat. Nos. 4,318,076; 4,241,604; 1,953;159; 1,496,486; and 1,342,860. None of the listed patents is concerned with windshear as such. The prior art does teach the detection and warning of wind velocity and direction but it does not direct itself in any way to detection and warning of windshear.